The present invention relates to a process for preparing high-purity 2,2-bis(4-hydroxyphenyl)propane (hereinafter referred to as bisphenol A).
Bisphenol A is used as a raw material for polycarbonate resins and epoxy resins. There is an increasing demand for colorless and high-purity bisphenol A (superior in quality to the conventional ones) which meets the requirements of polycarbonate resins for optical applications.
Bisphenol A is produced by reacting acetone with excess phenol in the presence of an acid catalyst and an optional co-catalyst such as a sulfur compound. The product mixture contains, in addition to the objective bisphenol A, the catalyst used, unreacted acetone, unreacted phenol, water and by-products.
The by-products contain, as major components, 2-(2-hydroxyphenyl)-2-(4-hydroxyphenol)propane (hereunder referred to as "o,p'-isomer") and Dianin's compound. Minor components thereof include trisphenol, polyphenols and coloring substances. They exert adverse effects on the performance of the resins produced from such bisphenol A.
There have been proposed many processes for removing these impurities (by-products) from the objective bisphenol A to obtain high-purity bisphenol A.
One example for obtaining high-purity bisphenol A from such a product mixture comprises the steps of removing the catalyst, unreacted acetone, water and a small amount of phenol from the mixture by vacuum distillation; cooling the residual liquid mixture to crystallize bisphenol A in the form of an adduct with phenol; separating the resulting crystals from the mother liquor containing the by-products; and removing phenol from the adduct to obtain high-purity bisphenol A.
The mother liquor from which the crystals are removed includes bisphenol A in addition to phenol and by-products and, therefore, it is recycled for reuse.
One example of such reuse thereof is to recycle the mother liquor to the reaction system. The o,p'-isomer and trisphenol which are principal components of the by-products exist in the reaction system at a constant equilibrium composition with bisphenol A and, therefore, a part of them may be recovered as bisphenol A. However, Dianin's compound, polyphenols and coloring substances exist in the reaction system as they are or react with the starting phenol or acetone to form high molecular weight substances and these substances remain in the reaction system. This leads to an accumulation of such substances in the reaction system due to the recycle of the mother liquor and in turn impairs the purity and color shade of the adduct.
For this reason, a part of the mother liquor recycled to the reaction system should be purged, but in this case useful bisphenol A would be disposed.
G.B. Pat. No. 1,565,667 and Japanese Pat. Publication No. 55-34779 disclose a method for recovering bisphenol A from the recycled flow of the mother liquor and for removing such coloring substances. According to this method, at least part of the recycled flow of the mother liquor is treated with an adsorbent consisting of a cation exchange resin to remove the coloring substances, prior to recycling it to the reaction system.
The effect attained by such a cation exchange resin as used in the invention disclosed in the foregoing patents can likewise be obtained in cases where other adsorbents are utilized. However, such adsorbents cannot be used continuously without any regeneration, the regeneration requires the steps of washing, drying and removing adsorbed substances (coloring substances) from the wash liquid and further the amount of the mother liquor which can effectively be treated and decolorized with such an adsorbent is not so great. Thus, it is required to frequently exchange the adsorbent. Moreover, not all the impurities present in the mother liquor are converted to bisphenol A and recovered. Therefore, the resulting bisphenol A necessarily contains impurities greater than those in bisphenol A obtained without recycling the mother liquor.
In addition, the recycle of the mother liquor to the reaction system results in the circulation of bisphenol A which has already been formed even if such recycle is performed in any manner. This leads to the substantial reduction in the productivity of the reaction system.
Moreover, U.S. Pat. No. 4,209,646 and Japanese Publication No. 52-46946 disclose a secondary process which comprises removing a part of the phenol in the mother liquor obtained in the principal process to concentrate it, further recovering crystals of the adduct of phenol with bisphenol A, using the recovered adduct or bisphenol A obtained by removing phenol from the adduct to prepare a liquid mixture, and then supplying the liquid mixture to the principal crystallization process. However, in this method, the mother liquor obtained in the secondary process is to be disposed. The mother liquor from the secondary process still contains not only bisphenol A, o,p'-isomer and trisphenol convertable to bisphenol A but also simultaneously contains polyphenols and coloring substances in high proportions. Thus, the mother liquor could not be recycled to any process.
Consequently, according to the conventional method, about 5% of the starting materials with respect to the total amount of bisphenol A product is innevitably disposed.
In G.B. Pat. No. 1,565,667 and Japanese Pat. Publication No. 55-34779, there is disclosed a method comprising treating a part of the residual mother liquor from which bisphenol A has been removed in the form of an adduct with phenol in the presence of an alkaline catalyst to cleave certain components present therein into phenol and p-isopropenylphenol and then recycling the cleavage product to the principal reaction process.
Likewise, U.S. Pat. No. 4,400,555 discloses a method comprising isomerizing a part of the residual mother liquor from which bisphenol A has been removed in the form of an adduct with phenol, or treating a part of the residual mother liquor in the presence of an acid catalyst to cleave certain components present therein into phenol and p-isopropenylphenol, and then isomerizing it and the untreated residual mother liquor, and thereafter recycling the cleavage products to the principal reaction process.
The cleavage reaction of the mother liquor containing o,p'-isomer or by-products conventionally employed is an effective tool for obtaining bisphenol A in a high yield, but it is difficult to prevent bisphenol A from being contaminated with low boiling substances other than phenol and p-isopropenylphenol. Thus, bisphenol A obtained by such a recombination shows purity lower than that of the product prepared from phenol and acetone. Under such circumstances, the yield of bisphenol A can be increased by recycling the cleavage product to the principal process, but on the contrary the load of the purification process increases and the gain due to the increase in yield correspondingly decreases.